The Prince of Wales personally arranged a Windsor walkabout with his wife, brother and sister-in-law after the death of Queen Elizabeth II, calling it “awkward” but the right thing to do for his beloved grandmother, a new biography has claimed.

The Waleses and Sussexes surprised the watching world when, despite being all-but estranged, they stepped out together to greet the mourning crowds.

A new book detailing the late Queen’s death and beginning of Charles III’s reign describes the “remarkable scene” at Cambridge Gate on Windsor’s Long Walk, after a black Land Rover pulled up and the two couples emerged.

“It was very much William’s idea,” the book by Robert Hardman claims, quoting one of Prince William’s closest advisors. “He had organised it in about two hours flat.

“He had been giving it a lot of thought and he said: ‘I know it’s awkward but isn’t it right in the context of my grandmother’s death?’

“I know he asked a couple of other people, too.”

After the walkabout, which lasted around 40 minutes and saw the couples speak to different parts of the crowd, a member of the Waleses team allegedly said: “I don’t think either couple found it easy.”

The episode underlines the extent of estrangement between Princes William and Harry at the time, ahead of the latter’s tell-all memoir and Netflix series.

The book, Charles III, relays how the new King and his elder son dined together at Birkhall after the Queen’s death, while the remainder of the senior Royal family stayed at Balmoral.

“The King needed to have vital but discreet discussions with his elder son,” the author says. “In years gone by, such a moment would automatically have included his younger son, too.

“But not any more. This was clearly not an occasion for an opening up of hearts and minds with Prince Harry, particularly if he was still taking notes for his forthcoming book.

“Charles III needed a clear head and no distractions.”

In a second walkabout described in the book, the King arrives back at Buckingham Palace to find emotional members of the public asking him for a hug.

The moment was so striking to the King’s aides that Sir Clive Alderton, private secretary, later told staff that the new reign would be characterised by the sort of “informal formality” already on display.

“He had been struck by the woman who had given the King a hug,” Hardman writes.

“To Sir Clive, this was a moment that crystallised the change of reign: ‘This reign will have an informal formality to it,’ he told the staff.”